1. Field of the Invention
The present invention relates generally to a system and method for providing high performance quantizer processing, and specifically to a system and method for providing high performance quantizer processing of sensor outputs in an inertial navigation system.
2. Description of Related Art
Aircraft inertial navigation relies upon the integration of data in order to achieve accurate tracking of the parameters of interest. The inertial navigation system of an aircraft includes various sensors, including accelerometers and gyroscopes, that convert the effects of inertial forces into acceleration, velocity and position measurements. The accelerometers determine acceleration forces along three orthogonal sensitive axes and this data is converted, through integrations, into the aircraft's velocity and position. The sensed acceleration is integrated to velocity in hardware, and the change in velocity across a predetermined interval of time is the value that is sampled by the inertial navigation system computer.
Due to the fact that the integrator will become saturated when its voltage limits are exceeded, it is impossible to integrate for an indefinite period of time without exceeding the voltage limits of the integrator. A quantizer is commonly used to integrate the instantaneous analog current (proportional to the instantaneous acceleration) output from an accelerometer, which is in turn used to measure the change in velocity. To prevent saturation of the integrator from occurring, the quantizer applies current resets to the integrator in order to keep the voltage bounded within its limits. These resets do not actually reset the integrator to zero, as this would cause a loss of velocity information, but instead the resets add or subtract (depending on polarity) a very precise amount of charge from the integrator.
A quantizer typically includes two constant current sources (a positive and a negative) connected to the integrator in order to respectively apply these precise amounts of charge to the integrator. The current sources apply fixed values of current, which enables a precise amount of charge to be added or subtracted by precisely controlling the amount of time a switch connected to a respective current source is closed to allow current flow. As long as the amount of reset charge is known precisely, the integrator output can be adjusted to accurately account for these resets in the system computer. Certain differences and errors in the applied positive and negative resets may provide sources of error in the integrator output.
Error sources that affect the accuracy of the integrator output require compensation to ensure accuracy of the navigation system measurements and functions. Because inertial grade instruments are required to measure a very large dynamic range of motions, they typically rely on state of the art technologies and must be able to measure extremely small quantities. For example, a navigation grade accelerometer must measure a few millionths of the standard gravity acceleration. Even the smallest errors can yield inaccurate results, where errors, such as bias, appearing in a quantizer are strongly dependent upon temperature and other sources. One possible source of error appearing in the integrator output results from the presence of a capacitor in the integrator hardware, where the capacitance (C) of the capacitor variably fluctuates proportionally with fluctuations in temperature. This capacitance (C) is used to formulate a voltage scale factor (1/C), which is used to convert the voltage output of the integrator to a desired measurement unit (i.e., velocity). Since the precise amount of reset charge being applied is known, it is also necessary to determine the precise value of the voltage scale factor in order to accurately determine the output of the integrator.
A quantizer typically includes a positive current source and a negative current source for applying respective resets, where it is possible for an asymmetry to exist between the positive and negative reset currents. This asymmetry causes the integrator output to drift in a direction corresponding to the unequal reset currents being applied, which will lead to inaccurate control of the integrator. Another possible source of error arises from a value of quantizer bias resulting from leakage currents being fed into the integrator. In order to account for such errors, it is desirable to provide for periodical calibration of the quantizer to detect and account for any asymmetry in the reset currents, voltage scale factor errors, and bias from leakage currents.
Typically, three accelerometers are provided in an INS for determining acceleration forces acting in three orthogonal axes, so the quantizer includes a respective channel for each direction of measured acceleration. In order to maintain three continuously operating channels for constantly measuring acceleration in three orthogonal directions, an extra fourth channel must be provided for calibration purposes. Whenever there is a need to calibrate one of the channels, the extra channel which is not being used to measure acceleration is switched with the channel requiring calibration. In this manner, three of the four channels are continuously operating in a data mode with the remaining channel operating in a calibration mode, where one of the channels is variably selected to be calibrated based on the operation of the quantizer.
A quantizer having multiple channels must be able to apply positive and negative resets to each of the channels to prevent all of the channels from becoming saturated. It is possible to provide a pair of current sources for each respective channel to apply these positive and negative resets, but requiring such a large number of current sources would greatly add to the cost of the quantizer as well as adding to the complexity of its circuitry. Thus, it is desirable for a quantizer to possess merely a single pair of current sources which are multiplexed between the channels for providing resets to the channels. The problem with existing quantizers which utilize only one pair of current sources for providing positive and negative resets to all of the channels is that the channels are merely cyclically connected to the current sources for resets to be applied without taking into account which channels are actually close to their saturation point and require application of a reset value.
There is clearly a need for a system and method for providing an improved quantizer processing scheme for determining which channels in a multi-channel quantizer are close to saturation and prioritizing the order in which the channels shall have resets applied thereto to prevent saturation of each of the various channels. Furthermore, there is a need to provide an improved and more robust calibration scheme for a multi-channel quantizer to accurately account for multiple sources of quantizer error in each of channels during in the field operation of the quantizer.